It is well known that boundary layer flow over a surface may be altered to change the boundary layer flow and resulting macroscopic parameters such as drag in the case of an aerodynamic body or flow energy loss in pipe flow. Controlling boundary layer separation for example, can be used to reduce form drag and control the unstable pressure fluctuations associated with separation. Separation increases the drag on the body due to the altered distribution of pressure when the flow separates. Preventing or delaying separation may reduce this drag. Delaying separation or inhibiting separation can also reduce pressure fluctuations.
Depending on the fluid, boundary layer control may be accomplished using any of a variety of methods. When the fluid is a gas, the usual method of altering the boundary layer is to increase or decrease pressure at the surface by injecting (or withdrawing) the fluid through ports formed through the surface. Another method is to introduce a different fluid into the stream adjacent the surface. Still other methods use magnetic fields to alter the flow of fluids having certain electromagnetic properties. Combinations of these methods may also be used.
One problem with all the above methods of altering boundary flow, is that they are generally difficult to use effectively in unsteady flow regimes. Most often, the use of boundary layer control devices is determined based on an overall flow regime. For example, boundary layer control on a wing structure may be based on overall flow parameters such as angle of attack and freestream velocity. The actual control inputs to the boundary layer control device are empirically determined for various flow conditions and are then applied based on measurement or calculation of these parameters in flight. This approach has some inherent drawbacks. For example, it does not account for differences in behavior at different points on a three dimensional structure. Moreover, it does not provide for real time control in unsteady flow regimes. As a general matter, there has not been a practical system for controlling boundary layer characteristics in unsteady, multidimensional flow regimes.